Seismic data acquisition systems conventionally use cabled networks comprising electronic units whereto ground movement sensors are connected.
FIG. 1 illustrates schematically a seismic data acquisition system according to a first known solution, based on the use of analog sensors 4.
For the sake of simplification, each reference 4 designates an analog sensor and its corresponding housing and casing (as detailed below with FIG. 2).
To collect the seismic data (geophysical data), one or a plurality of seismic sources (not shown in FIG. 1) in contact with the ground are activated to propagate omni-directional seismic wave trains. The sources may among other things consist of explosives, falling weights, vibrators or air guns in marine environments. The wave trains reflected by the layers of the subsurface are detected by the analog sensors 4, which generate an analog signal characterising the reflection of the waves on the geological interfaces of the subsurface.
The analog sensors 4 are generally referred to using the term “analog geophones”. As shown in the example of FIG. 2, they are generally interconnected in groups of sensors by a two-conductor line 5 (or a three-conductor line for a serial-parallel configuration) to form clusters referred to as “strings of analog geophones” (or “geophone strings”) 33. To this end, each analog geophone is mounted in a mechanical housing (or cartridge) 62. This mechanical housing 62 of analog sensor is inserted with mechanical tolerances inside a casing 61 (made of plastic in general) which shape is dependent of the type of area to investigate (marsh, land . . . ). The two-conductor line 5 is usually moulded to the casing 61. Each of the strings 33 is connected to an acquisition device 3. To this end, the acquisition device 3 is also mounted in a mechanical housing which comprises a connector 63 with two contacts, adapted to cooperate with a connector 64 of the same type placed on the two-conductor line 5 (i.e. the cable of the string).
The acquisition devices 3 are generally referred to using the term “Digitizer Unit”. They are interconnected wirelessly or by a cabled network (e.g. a four-conductor line), perform the analog to digital conversion of analog signals coming from the groups of sensors and send the resulting digital seismic data to a central recording system 1 (also referred to using the term “central data processing unit”), via intermediate collection devices 2 (also referred to using the term “concentrator device”). The central recording system 1 is usually onboard a recording truck. The acquisition devices 3 also performs other functions, notably: synchronisation with the central recording system 1, processing of the seismic signal and interfacing with the digital network (i.e. transferring seismic data to the central recording system 1, receiving and processing commands received from the central recording system 1).
The arrangement of analog geophones to form a geophone string 33 depends on the useful signal level noise level and the level of spatial filtering required, which themselves depend on the soil type and varies from one mission to another.
It is common practice to assemble, in series and/or in parallel, a plurality of geophones to form a geophone string 33, since:                geophones connected in series allow to amplify the signal (more the signal is weak, the greater the number Ns of geophones in series must be; however the number Ns should not be too great, not to overload the input);        geophones connected in parallel allow to get a low impedance and be immune to electrical noise.        
A serial or parallel configuration of a plurality of geophones allows to filter the noise (the more you want to filter the noise, the greater the number of geophones in series Ns and/or parallel Np must be; the limit is a compromise between cost and performance).
In a first example, a geophone string 33 comprises Ns geophones in series. The resistance of this geophone string 33 is: Rstring=Rgeo×Ns, with Rgeo the resistance of one geophone.
In a second example, a geophone string 33 comprises Np geophones in parallel. The resistance of this geophone string 33 is: Rstring=Rgeo/Np.
In a third example, a geophone string 33 comprises Np branches in parallel, each branch comprising Ns geophones in series. The resistance of this geophone string 33 is: Rstring=Rgeo×Ns/Np.
Many other examples of geophone strings can be considered.
For reasons of ease and speed of implementation, the aforesaid connectors 63, 64 (used to connect each geophone string 33 to an acquisition device 3) do not have locking ring. To achieve a spatial filtering, the set of geophones (analog seismic sensors) which are connected (via one or several geophone strings) to an acquisition device are commonly deployed on hundreds of m2 and are therefore subject to numerous mechanical constraints (vehicles, animals . . . ) and climatic constraints, which can induce an unintentional disconnection of all or part of the geophone strings, or a break of the cable connecting the geophones together and to the acquisition device. It is therefore necessary to provide the system operators with a solution to remotely monitor the correct connection of the geophone strings, in order to allow maintenance or warn about the poor quality of the data produced by the acquisition device. This warning information can be used during the processing of the complete data.
A known solution is now described with FIGS. 3 and 4 in which a geophone string 33 is connected to an acquisition device 3, and the acquisition device is configured either in acquisition mode (FIG. 3) or in test mode (FIG. 4). In this example, the geophone string 33 comprises five geophones 4 connected in parallel.
The acquisition device 3 comprises:                a pair of input terminals 38a, 38b to which is connected the geophone string 33;        a passive filter 34 for filtering the noise;        an analog-digital converter (ADC) 35;        a processor 36 (e.g. a microprocessor or a microcontroller);        a current generator 37a, 37b (shown only in FIG. 4, since it is used only in the test mode).        
In the acquisition mode illustrated in FIG. 3, the voltage U between the input terminals 38a, 38b represents an analog useful seismic signal generated by the set of geophones 4 of the geophone string 33. Said useful signal coming from geophones comprises no offset signal. The processor 36 processes the signal 310 resulting from the filtering (by passive filter 34) and the analog to digital conversion (by the ADC 35) of this analog useful seismic signal, and generates a digital useful seismic signal 39.
In the test mode illustrated in FIG. 4, a current I is injected by the current generator 37a, 37b into the geophone string 33. The voltage U between the input terminals 38a, 38b represents an analog test signal which depends on an equivalent resistance Req: U=Req*I, with Req a resistance equivalent to Rstring in parallel with Rin, with Rstring the resistance of the entire set of geophones 4 of the geophone string 33 and Rin the input resistance of the acquisition device 3:
  Req  =            Rin      ×      Rstring              Rin      +      Rstring      
The processor 36 processes the signal 41 resulting from the filtering (by passive filter 34) and the analog to digital conversion (by the ADC 35) of this analog test signal, and generates a value 40 of Rstring (since I and Rin are known, the value of Rstring can be deduced).
The above known solution (illustrated in FIGS. 3 and 4) has several drawbacks detailed below.
Measuring the resistance Rstring (by current injection) cannot be carried out in operation, i.e. during an acquisition, because this measuring interferes with the geophones (because injected current I overrides useful seismic signal from geophones). Therefore the measuring of the resistance Rstring is usually performed only twice a day and when seismic data acquisition system is in standby between two seismic acquisitions. However, acquisition systems are increasingly used in continuous acquisition (24 h/24 h), which does not allow performing regular resistance test, in order to detect continuously any accidental disconnection of all or part of the geophone strings 33 that would lead to a drastic degradation of seismic signal quality.
Because of changes of the resistance Rstring as a function of the temperature and because of the disparity in the resistance values, the known solution can lead to false alarms and provides good results only by comparing the resistance value obtained by a given acquisition device (to which is connected e.g. one geophone string) with the average value of the measured resistance values obtained by other acquisition devices (to each of which is connected e.g. one other geophone string).
In the known solution, an absolute measuring of the resistance Rstring is made. This absolute measuring must be precise to discriminate the correct number of connected geophones. Therefore, to be robust to noise, it is necessary to inject a high level current. For example, if we consider a geophone string having a typical configuration with six geophones in parallel, with the aim to continuously check that there is six geophones connected (i.e. a geophone string resistance: Rstring=Rgeo/6, with Rgeo the resistance of each geophone), we must be able to measure the resistance Rstring of the geophone string with an accuracy of Rgeo/6 and compare the measured resistance value to a theoretical resistance value. This means being very sensitive to noise, and therefore use a high level current.
In addition, if one includes the effects of temperature on the value of the resistance (+/−20% of the range of the resistance), the comparison with a theoretical resistance value is impossible. It is therefore necessary to make a comparison with the measured resistance values obtained with all the other acquisition devices, which is difficult and even impossible in the context of autonomous acquisition devices.